The present invention relates to toners useful in electrostatographic processes and, more particularly, to toner compositions providing fused toner images having controlled gloss.
In a fuser such as that used in the NEXPRESS 2100 printer, a smooth surfaced fusing roller is used to apply heat and pressure to an unfused toner image on a receiver sheet such as a clay-coated paper stock. The toner particles are fused together and adhered to the receiver sheet, and become spread out to a certain degree. The top surface of the toner deposit so produced is characterized by a degree of smoothness that can be quantified with a gloss measurement. The degree of gloss itself is important to the perception of quality of the image, and to measurable aspects such as reflection density and degree of color saturation. For a given degree of spread of the toner (measured for a specified area of white paper covered by colored toner), an increase in gloss will result in increases in reflection density and in color saturation. It is observed that, in general, as the temperature of the fuser roller is increased, the degree of gloss increases. The slope of gloss versus temperature is, however, quite steep, making it difficult to reproduce a desired gloss level on a print-to-print basis, or even within an individual print basis, because of inherent difficulties in controlling temperature fluctuations in roller fusing systems. These difficulties include, among others, fuser temperature drop in an extended run of prints resulting from heat removal by the paper, temperature overshoot when printing is temporarily stopped, temperature sensor variability, mechanical tolerance difficulties leading to greater nip width at one end of a roller compared to the other, fuser roller surfaces of varying smoothness resulting from wear or manufacturing variability, and, notably, paper stocks of variable heat capacity and water content.
It has been observed in toner/fuser systems that, for paper stocks of the glossier variety, low density areas of the toner image have a lower degree of gloss than areas of the print having higher toner laydown. It would be desirable to find toner compositions that would exhibit less of this so-called differential gloss phenomenon. Although high gloss prints have very high densities and color saturation, it is commonly perceived that they are less pleasing and of lower quality than images of a controlled mid-gloss level. Images with satin appearing gloss in the range of 10 to 40 units of the Gardiner 60 degree angle scale (G60 gloss) are generally preferred to shiny images with higher G60 values. Therefore it would be desirable to provide toner compositions that would readily and reproducibly produce gloss values in the desired range in the fusing system of an electrostatographic printer.
It has now been found that dry blending toner particles that have been separately prepared with a lower melt viscosity resin with toner particles that have been separately prepared with a higher melt viscosity resin produces a blended toner that manifests a substantially reduced slope of gloss versus temperature, compared to either of the pure high or low viscosity toners comprising the blend, over the mid-gloss range of interest. Such blended toners have been found to yield a lower degree of differential gloss, and provide an easy way to prepare a toner that, by selection of a blend of the proper ratio of the blend, will produce gloss values in the desired range.
The preparation of toners using blended high and low melt viscosity resins within the same toner particle is known in the art. For example, U.S. Pat. No. 4,246,332 describes the preparation of toners by melt blending a non-offsetting, high molecular weight, low fluidity styrene-acrylic resin with a high fluidity polyester or epoxy or vinyl resin in order to improve low temperature fixability. U.S. Pat. No. 5,082,883 describes a low viscosity epoxy resin melt blended with a higher viscosity polyester to produce a toner that has lower viscosity than the polyester itself, which allows low fusing temperature, but still retains some of the elastic character of the higher molecular weight branched polyester, which is desirable for conferring anti-offset properties to the toner. U.S. Pat. No. 5,156,937 describes toners comprising melt-blended low and high molecular weight polyesters that fuse at low temperatures and times characteristic of the low viscosity component, but retain enough of the melt cohesive strength of the high viscosity component so that substantially all of the toner remains adhered to the paper during hot roller fusing and thus does not offset. U.S. Pat. No. 5,518,848 describes toners prepared from melt-blended high and low molecular weight resins of specified monomer compositions in order to realize good fixing along with blocking resistance and anti-offset properties. U.S. Pat. No. 5,556,732 describes the preparation of toners by melt-blending a higher viscosity xe2x80x9clow gloss valuexe2x80x9d polyester with a lower viscosity xe2x80x9chigh gloss valuexe2x80x9d polyester in order to achieve a toner with a gloss value intermediate to that of the pure components at a given fusing condition. U.S. Pat. No. 6,168,894 describes a toner composition formed by melt blending of a high viscosity polyester resin, sufficiently cross-linked to have an insoluble component, into a low viscosity polyester resin, wherein the high viscosity resin is phase separated within the low viscosity resin. The improvement cited is the achievement of a wide fixing range without offset.
However, since the toners of all of the aforementioned patents, the disclosures of which are incorporated herein by reference, have the same melt characteristics and composition on a particle to particle basis because of the melt blending step in their preparation, they all suffer from the difficulty of controlling gloss level due to the steepness of the gloss versus fusing temperature relationship or gloss versus fusing time relationship. It is the purpose of this invention to provide a toner having reduced sensitivity of gloss to fusing temperature and time variations.
The present invention is directed to a particulate toner composition comprising a combination of a low viscosity polymeric particulate toner component having a first selected melt viscosity and a first selected melt elasticity, and a high viscosity polymeric particulate toner component having a second selected melt viscosity and a second selected melt elasticity. The first and second melt viscosities and first and second melt elasticities are each selected so as to produce a lower variation in measured G60 gloss values as a function of fusing temperature for fused images formed from the combination of particulate toner components than the corresponding variation in measured G60 gloss values for fused images formed from the low viscosity polymeric particulate toner component of the composition.
The present invention is further directed to a process for forming a particulate toner composition that comprises combining a previously prepared low viscosity polymeric particulate toner component having a first selected melt viscosity and a first selected melt elasticity with a separately prepared high viscosity polymeric particulate toner component having a second selected melt viscosity and a second selected melt elasticity. The resulting toner composition provides fused images having controlled gloss characteristics.
Also in accordance with the present invention is a process for forming a fused toner image that comprises: forming on a receiver sheet an unfused toner image of the disclosed particulate toner composition, and heating the unfused toner image to a fusing temperature sufficient to form a fused toner image that, preferably, has a G60 gloss value of about 10 to about 30 on the receiver sheet.